1. Field of the Invention
This invention relates to a method for analyzing the anisotropic properties of ferromagnetic steels; more particularly, to a method which measures the anisotropic properties of such steels by use of Barkhausen noise simultaneously sensed in a plurality of directions without contact with the steel being analyzed.
2. Description of the Prior Art
In the processing of steel, it has long been the desire of workers in the art to analyze the properties, both chemical and physical, of the steel at an early stage in its formation to determine whether the steel will be suitable for its intended purpose. Chemical analysis has been advanced by a number of process improvements including more careful selection of charging materials, off-gas analysis in the basic oxygen furnace and the like. The measurement of physical properties, however, has continued to be essentially a sampling procedure where, for example, a piece of strip steel is removed from a coil after cold-rolling and tested for the physical properties of interest. This "off-line" testing suffers at least two drawbacks: First, the properties of the test piece may not be representative of the entire coil, i.e., the testing is random. Second, little opportunity is afforded for making "upstream" corrections, for example in a cold-rolling operation, to cure defects resulting from process conditions being applied there.
Accordingly, a need exists for an accurate technique for "real-time" testing of the physical properties of steel, i.e., as it is being processed, as well as better assurance that test results are representative of the physical properties of an entire sheet or coil of steel. The former would reduce the amount of in-plant scrap generation; the latter, of course, would dramatically reduce the instances of customer rejection of products because of the inexactness of random sampling methods now employed.
The type of testing under consideration here is called nondestructive testing (NDT). Well-known NDT methods are, for instance, eddy current and ultrasonic analysis, and possibly X-ray diffraction. The eddy current method has been successfully used for fast inspection of surface cracks and material characteristics. Ultrasonic methods work well for internal crack and void detection, and attempts have been made to employ ultrasonic methods for stress measurement. X-ray diffraction is considered to be too slow and complicated for on-line inspection. Another method of nondestructive testing of ferromagnetic steels involves the use of Barkhausen noise or the Barkhausen effect. An excellent review of this subject appears in a paper entitled "The Barkhausen Effect And Its Application To Nondestructive Evaluation" published in October 1979 by the Nondestructive Testing Information Analysis Center, Southwest Research Institute, San Antonio, Tex. As that article points out, when a magnetic field of slowly varying intensity is applied to a ferromagnetic material, an interaction occurs at the atomic level in small regions called domains; specifically, the walls of these domains are set in motion and when they come into contact with impediments to this motion such as dislocations, grain boundaries, precipitate particles, etc., a jump or pulse in magnetization occurs. These abrupt movements of the domain walls generate a crackling sound which may be sensed by appropriate acoustical equipment and is called Barkhausen noise. It is now known that Barkhausen noise may be correlated to material properties such as stress, grain size, grain form and orientation (hereinafter described as "texture"), and phase morphology. One such correlation technique is discussed in U.S. Pat. No. 3,427,872.
The present invention employs the known principles of Barkhausen noise to achieve a new, practical technique for the nondestructive testing of ferromagnetic steels to determine their anisotropic properties. For a better understanding of this invention, it is desirable to discuss anisotropy in greater detail.